Project 3 - SUMMARY: Groundwater arsenic contamination is both pervasive and difficult to remediate. Our previous research indicates that in situ magnetite formation forms a diffuse barrier capable of long-term As retention. The overarching goals of this project are to develop an enhanced remediation technology that produces magnetite in situ to form a reactive barrier that sustains low arsenic concentrations both in the laboratory and in field trials. Our aims include a series of laboratory-based and field-based experiments to (1) optimize and (2) implement magnetite-based remediation approaches in the field. The laboratory-based aims are designed to develop an improved understanding of the fundamental processes to maximize the potential of magnetite- based approaches to remediation. This information can be used to develop models that account for mineralogical and solution composition changes that effectively predict As fate and transport. We will then use these models to design pilot remediation efforts at the USGS research site on Cape Cod and the Lot 86 Superfund site on the NC State University. Field-based investigations at these two heterogeneous contaminated sites will characterize both background conditions and magnetite formation mechanisms to upscale laboratory data, refine reactive transport models, and develop a ?remediation toolbox? to facilitate robust designs of site-specific remediation strategies.